1. Technical Field
The embodiments herein generally relate to electrical components, and, more particularly, to analog-to-digital converters (ADCs).
2. Description of the Related Art
Class-D is a switching-based audio amplifier technology and theoretically it can achieve approximately 100% power efficiency. Traditional Class-AB audio amplifiers typically have much lower efficiency. FIG. 1 shows a typical power efficiency comparison between a class-D amplifier and a class-AB amplifier. For portable devices such as MP3 players and multi-media cell phones, etc., where the battery power consumption is critical, users are motivated to use more power efficient class-D audio amplifiers to replace the current market-dominant class-AB amplifiers, which only has approximately 20-30% efficiency for portable digital audio applications.
The major technology barrier to overcome in order to make a good quality class-D audio amplifier is to provide a sufficient power supply rejection ratio (PSRR). Class-D uses a pair of metal-oxide-semiconductor field-effect transistor (MOSFET) power transistors, switched on and off by the input pulse-width modulation (PWM) waves, to drive the speaker load, as shown in FIG. 2. If no measure is taken; i.e., PSRR=0 dB, any VCC power rail ripples will directly propagate to output load, thus destroying the signal-to-noise ratio/total harmonic distortion (SNR/THD+N) performance of the audio amplifier. A high-end audio amplifier is usually specified with >90 dB SNR. Considering the power rail noise level is in the order of 10 mV (−40 dB), a PSRR of approximately 50 dB generally must be provided in order to deliver 40 dB+50 dB 90 dB SNR.
Currently, the commercially available class-D amplifiers are generally all analog-type as shown in FIG. 3. The PSRR is provided by the analog feedback loop which corrects the distortion on the analog output by comparing it with the ideal analog input signal through a feedback loop filter. Typically, these analog class-D audio amplifiers can only work in high-power (>10 W) applications such as home theatres and flat panel televisions with good power efficiencies (approximately 90%). For low-power portable device applications, where the audio output power is typically in the range of 10-100 mW, the analog class-D power efficiency drops to <30% due to overhead analog circuitry of analog class-D; i.e., the feedback loop and an external digital-to-analog converter (DAC) to convert the digital input into analog input. Currently, for portable digital audio applications, there are no suitable commercial class-D audio amplifiers which can deliver high power efficiency.
Besides the analog approach, there is a digital approach to provide PSRR for class-D; i.e., digitalizing the power supply ripple and then using digital signal processing techniques to correct output distortions caused by power supply ripple. To make this approach work for class-D audio amplifiers for portable applications, a high-speed, low-power consumption, and high-resolution ADC needs to be provided in order to digitize the voltage ripples on power supply. Because digital circuits consume much lower power, such digital class-D devices can deliver much higher power efficiency for portable applications.
Conventional ADCs for audio applications use analog sigma-delta modulators based on either switched-capacitor integrator circuits or continuous-time integrator circuits that consume relatively high power. Therefore, the conventional ADCs cannot be used for building a digital class-D audio amplifier with a suitable PSRR performance for portable applications.
As such, those skilled in the art would readily acknowledge that there remains a need for a novel type of ADC which has low-power consumption and high speed in order to build a power-efficient class-D audio amplifier with sufficient PSRR performance.